In analog circuitry, signals are often represented as differential voltages. A differential signal may be characterized as having a differential component and a common-mode component. A differential amplifier is very useful in processing differential signal voltages. If the differential amplifier also provides a differential output signal, it is known as a fully differential amplifier. The fully differential amplifier needs a mechanism to set the common-mode voltage to a desired reference voltage, such as an analog ground voltage, typically designated "V.sub.AG ". This mechanism is commonly a circuit which senses the common-mode component of the fully differential output signal, and provides a feedback signal to the amplifier proportional to the sensed difference between the two signals. This common-mode feedback mechanism thus prevents the common-mode voltage from drifting away from the desired reference voltage.
Known differential amplifiers are able to function with characteristics close to those of ideal voltage amplifiers, by having high input impedance, low output impedance, and stable frequency response. For some circuit applications, however, amplifier operation may become unstable. For example, some common-mode feedback circuits cause a node in the feedback circuit to compete with the output nodes of the amplifier gain stages as the dominant pole of the amplifier, causing instability in known amplifiers. Thus, new amplifiers are needed for this and other instability problems.